Nick is one cynical little cookie. Even though Nick reserves explicit judgment on the characters, Fitzgerald still manages to implicitly criticize through his narrator's tone. (Think about how ludicrous Myrtle seems when, although she isn't upper class, she still tries to look down on her husband.) Let's take a look at two passages. This first one is from Chapter 1, when Nick is hanging out with the Buchanans and Jordan for the first time:
"I love to see you at my table, Nick. You remind me of a – of a rose, an absolute rose. Doesn't he?" She turned to Miss Baker for confirmation: "An absolute rose?"
This was untrue. I am not even faintly like a rose. She was only extemporizing, but a stirring warmth flowed from her, as if her heart was trying to come out to you concealed in one of those breathless, thrilling words. Then suddenly she threw her napkin on the table and excused herself and went into the house.
Nick may be aware of the ridiculousness of certain social circumstances, but he's also aware of the seductive quality of the upper class. The tension between the two produces this cynical tone, where it's almost as though he's mocking himself for being taken in by it. "Untrue," he says: "I am not even faintly like a rose." At the same time, he responds to her words, seeing them as "stirring" and thrilling."
In the end, Nick passes along his judgment as the absolute truth. For instance, take a look at this excerpt from the last few pages of the novel, when Nick has become disillusioned with his former acquaintances:
I couldn't forgive [Tom] or like him, but I saw that what he had done was, to him, entirely justified. It was all very careless and confused. They were careless people, Tom and Daisy – they smashed up things and creatures and then retreated back into their money or their vast carelessness, or whatever it was that kept them together, and let other people clean up the mess they had made....
I shook hands with him; it seemed silly not to, for I felt suddenly as though I were talking to a child. (9.143-44)
Nick sums us Tom and Daisy as "careless." We don't get to decide for ourselves (or, at least, Nick doesn't want us to); we're just supposed to believe him, and believe his interpretations of events. But we know that Nick already feels torn about these people, torn between hard-nosed cynicism and romantic indulgence. Can we really trust him?